1. Field of the Invention
The present invention relates to a laser processing method and a production method of multilayer flexible printed wiring board using the laser processing method. Particularly, the present invention relates to a laser processing method of forming a via hole by removing multiple materials of different workability and a production method of multilayer flexible printed wiring board using the laser processing method.
2. Background Art
In recent years, as electronic devices are downsized and have higher functionality, there has been a growing demand for an increased density with respect to a printed wiring board. To realize a printed wiring board capable of high-density packaging, a buildup printed wiring board is used. Such buildup multilayer flexible printed wiring board is generally formed by using either a double-sided printed wiring board or a multilayer printed wiring board as a core board and providing one or two buildup layers on both surfaces or one surface of this core board.
Further, to increase the package density of the printed wiring board, a via is provided on the buildup printed wiring board for interlayer connection. The via is an interlayer conduction path including a plated layer formed on the internal wall of a via hole.
Types of via type include a skip via and a step via, in addition to a normal via (hereinafter referred to as “simple via”) connecting wiring patterns of two adjacent layers. The skip via is formed by plating the internal wall of a skip via hole. The step via is formed by plating the internal wall of a step via hole (i.e. stepwise via hole) in which the diameter becomes smaller in a deeper internal layer of the printed wiring board (for example, see Patent Literature 1).
When first, second and third wiring patterns are formed in this order through the intermediary of an insulating film in the thickness direction of the multilayer flexible printed wiring board, the skip via skips the second wiring pattern and electrically connects the first wiring pattern and the third wiring pattern. Meanwhile, the step via enables high-density interlayer connection by collective interlayer connection of the first, second and third wiring patterns.
The via hole can be formed by a conformal laser processing method of using a mask hole (i.e. conformal mask) provided on a conductive film on a processed layer and removing a processed layer exposed in this mask hole by laser light.
However, in the related art, when the processed layer is formed of multiple materials of different workability, for example, since a resin remains in the via hole, it is difficult to form a via of high reliability. This problem will be explained in more detail with reference to the drawings.
FIG. 3A and FIG. 3B are process cross-sectional views illustrating a production method of a multilayer flexible printed wiring board in the related art.
(1) First, there is prepared a double-sided copper-clad laminate having copper foils 152 and 153 on the both surfaces of a flexible insulating base member 151 formed of a polyimide film. By processing these copper foils on the both surfaces of this double-sided copper-clad laminate into a predetermined pattern by a photofabrication method, a circuit base material 156 illustrated in FIG. 3A(1) is acquired.
Mask holes 154a, 154b and 154c are formed on the copper foil 152 on the top surface of this circuit base material 156. Also, an internal layer circuit pattern 155 including a mask hole 154d is formed on the copper foil 153 on the back surface of the circuit base material 156.
(2) Next, there is prepared a coverlay 159 in which an adhesive layer 158 (having a thickness of 15 μm)) formed of an acrylic or epoxy adhesive is formed on a polyimide film 157 having a thickness of 12 μm. Subsequently, for example, using a vacuum press or vacuum laminator, the coverlay 159 is attached to the back surface of the circuit base material 156.
Through the above process, a coverlay-attached circuit base material 160 illustrated in FIG. 3A(2) is acquired.
(3) Next, a circuit base material 166 is acquired by performing the same process as in above process (1) using another double-sided copper-clad laminate having copper foils 162 and 163 on the both surfaces of a flexible insulating base member 161. As illustrated in FIG. 3A(3), the copper foil 162 on the top surface of the circuit base material 166 forms an internal layer circuit pattern 164, and a mask hole 165 is formed on the copper foil 163 on the back surface.
(4) Next, an adhesive layer 167, which is acquired by cutting an adhesive film in accordance with the shape of the circuit base material 166, and the circuit base material 166 are aligned.
(5) Next, as illustrated in FIG. 3A(4), using a vacuum press, the circuit base material 166 and the coverlay-attached circuit base material 160 are laminated and bonded through the intermediary of the adhesive layer 167.
Through the above process, a multilayer circuit base material 168 illustrated in FIG. 3A(4) is acquired.
(6) Next, as illustrated in FIG. 3B(5), conformal laser processing is performed using a carbon dioxide laser (wavelength: about 9.8 μm). In this way, simple via holes 169 and 170, a skip via hole 171 and a step via hole 172 are formed. At the time of this laser processing, the mask holes 154a, 154b, 154c, 154d and 165 function as a conformal mask.
Through the above process, a multilayer circuit base material 173 illustrated in FIG. 3B(5) is acquired.
(7) As illustrated in FIG. 3B(6), a plated layer of about 20 μm is formed by applying electrolytic plating processing to the whole surface of the multilayer circuit base material 173, and, after that, a conductive film of a buildup layer is processed by the photofabrication method to form external layer circuit patterns 178A and 178B.
By forming a plated layer on the internal walls of the simple via holes 169 and 170, simple vias 174 and 175 are formed. By forming a plated layer on the internal wall of the skip via hole 171, a skip via 176 is formed. By forming a plated layer on the internal wall of the step via hole 172, a step via 177 is formed.
After that, although it is not illustrated, if necessary, the top surface of a land part or the like is subjected to surface processing such as solder plating, nickel plating and gold plating, and, in a part for which the solder plating is not necessary, a protective photo solder resist layer is formed. After that, outline processing is performed by punching and the like through the use of metallic mold.
Through the above process, a multilayer flexible wiring board 179 illustrated in FIG. 3B(6) is acquired.
Next, an explanation will be given to a problem caused at the time the skip via hole 171 and the step via hole 172 are formed by laser processing.
Processed layers at the time of forming the skip via hole 171 and the step via hole 172 are the flexible insulating base member 151, the adhesive layer 158, the polyimide film 157 and the adhesive layer 167 in this order from the laser radiation surface side.
The flexible insulating base member 151 and the adhesive layer 158 are different in the decomposition temperature and the absorbance in a wavelength of around 10 μm which is a waveband of a carbon dioxide laser. That is, the flexible insulating base member 151 has a higher decomposition temperature than that of the adhesive layer 158, while the adhesive layer 158 has a higher absorbance in the waveband of the carbon dioxide laser than that of the flexible insulating base member 151.
In other words, compared to the flexible insulating base member 151, the adhesive layer 158 arranged below the flexible insulating base member 151 has a lower decomposition temperature and a higher absorbance in the waveband of the carbon dioxide laser.
Therefore, by laser light thorough the flexible insulating base member 151, ablation occurs in the adhesive layer 158 earlier than the flexible insulating base member 151. In this way, as illustrated in FIG. 4(a), a gas caused by the ablation of the adhesive layer 158 deforms the flexible insulating base member 151 and causes a bulge part 180. After that, when the pulsed laser is further radiated, as illustrated in FIG. 4(b), the bulge part 180 bursts and a burr part 181 is caused.
When the burr part 181 occurs, processed layers below it (e.g. the flexible insulating base member 151 and the adhesive layer 158) are prevented from being subjected to laser processing. Therefore, a resin residue is likely to occur on the internal layer circuit patterns 155 and 164 exposed into the skip via hole 171 and the step via hole 172.
Especially, in the case of forming a step via hole, as illustrated in FIG. 4(b), due to the step configuration, part of the burr part 181 is likely to be placed on a copper foil of an external layer. In such a case, subsequent laser processing is largely blocked.
Such a resin residue is not removed even in a desmear process performed after a laser processing process. As a result, plating adhesion to the internal walls of the step via hole and skip via hole degrades, which is a cause of degrading the reliability of an interlayer conduction path.
Also, the same applies to the polyimide film 157 and the adhesive layer 167. That is, by pulsed light passing the polyimide film 157, the adhesive layer 167 having a lower decomposition temperature and higher absorbance than the polyimide film 157 is subjected to ablation earlier.
To remove the above resin residue, it is necessary to further radiate pulsed laser. However, when the number of shots of pulsed laser is increased, there arises a problem that the productivity decreases.
Also, when the number of shots is excessively increased, the heat is accumulated in the adhesive layers 158 and 167, and, as a result, as seen from FIG. 3B(5), the adhesive layers 158 and 167 largely pull back in the side walls of the skip via hole 171 and the step via hole 172, which increases the asperities of the side walls. As a result, as seen from FIG. 3B(6), discontinuous parts occur in plate layers formed on the side walls, which is a cause of degrading the via reliability.
While it is thus necessary to remove a resin residue, it is required to form a via hole in as few shots as possible in order to improve the productivity and maintain the via reliability.
Therefore, to reduce the number of shots required for processing, it is considered to increase the energy density per shot of pulsed laser. However, in a case where the copper foil 162 forming the internal layer circuit pattern 164 is thin (for example, equal to or below 12 the pulsed light penetrates the copper foil, which is a cause of an occurrence of short defect. In recent years, to form a fine wiring pattern, there are many cases where a copper foil has to be made thin.
Also, in the case of forming the step via hole 172, even after the flexible insulating base member 151 is removed and the internal layer circuit pattern 155 is exposed, to remove the adhesive layers 158 and 167 and the polyimide film 157, pulsed light of high energy density is radiated. Therefore, in a case where the copper foil 153 forming the internal layer circuit pattern 155 has a relatively thin thickness (for example, equal to or below 12 μm), the internal layer circuit pattern 155 (i.e. copper foil 153) deforms and there occurs a space part between the internal layer circuit pattern 155 and the adhesive layer 158. This space part causes a discontinuous part in the plate layer formed on the side wall of the step via hole, which is a cause of degrading the reliability of the step via.
Also, in the case of radiating pulsed light of high energy density per shot, similar to the case of increasing the number of shots, since the heat amount accumulated in the adhesive layers 158 and 167 becomes large, the retreat amounts of the adhesive layers 158 and 167 increase and the asperity of the side wall of the via hole becomes large.
Meanwhile, there are known two laser processing methods, that is, there are known a cycle processing method of radiating pulsed light in this order on a one-shot basis to a plurality of mask holes provided in a processing area and a burst processing method of sequentially radiating pulsed light to one mask hole.
In the case of the burst processing method, by radiating a laser pulse of the second shot before the flexible insulating base member 151 bulges and turns up by pulsed light of the first shot, and by removing the flexible insulating base member 151, there is a possibility of being able to suppress an occurrence of resin residue. However, to continuously radiate pulsed light to the identical part, the heat amount accumulated in the radiated part becomes large. As a result, similar to the case of increasing the energy density per shot and the case of increasing the number of shots of pulsed light of small energy density, there occurs a case where, for example, the asperity of a via hole side wall becomes large.
In the related art, to remove a resin remaining in a via hole, there is a known laser processing method of increasing energy density of pulsed light on the way of laser processing of the via hole (Patent Literature 2). In this method, the number of processed layers is only one (resin layer 12) and therefore there does not occur a phenomenon of the above bulge and burr of the flexible insulating base member. Also, in a case where a metal layer (i.e. copper land 11) is thin, there may occur a case where pulsed light of large energy density causes the penetration and deformation of the copper land 11 exposed in the bottom surface of the via hole.
In addition, to solve the above problem, as a material of the adhesive layers 158 and 167, it is considered to use a polyamide adhesive having a lower laser light absorbance and higher decomposition temperature than an epoxy or acrylic adhesive. However, the polyamide adhesive is more expensive than the epoxy or acrylic adhesive which is generally used for a multilayer flexible printed wiring board. Therefore, there is a problem that the production cost increases.